The present invention relates to a multiple-stage centrifugal pump including a controlled leakage hydraulic balancing drum.
As is known, centrifugal pumps are susceptible to generate, in their operation, depending on their impeller being either hydraulically balanced or not, a reactive axial force between the rotary shaft/impeller portion and the fixed body/support/bearing portion.
In multi-stage centrifugal pumps, including several series-arranged pump impellers, said axial reactive force can achieve high values and, since it would be undesirable to directly transmit this force exclusively to the pump shaft support, different types of balancing devices therefor have been designed, which balancing devices, in particular, comprise balancing disc and balancing drum devices.
In a balancing disc/counter disc system, the axial load is transmitted to a disc coupled to the rotary shaft which can be freely axially driven with respect to the pump body and supports.
At the end of the pump body, on the pushing side thereof, is assembled a fixed disc element which, together with the rotary disc element assembled on the pump shaft, provides a radial leakage path therethrough the pump fluid flows.
In operation, a contactless narrowing, based on the pressure difference between the disc inner side and outer side, is formed, thereby transmitting the axial load under constant flow-rate/pressure head operating conditions. However, the designing of this balancing system is very difficult, in particular for variable pressure heads and flowrates.
In the balancing drum system, on the other hand, the axial load is transmitted to a balancing drum coupled to the pump shaft rotating in a static bush arrangement in the pump body cavity with a minimum radial clearance.
The pressure difference between the drum inner portion and outer portion is such as to balance the pump rotor, the residual load being absorbed by a thrust bearing axially locking the rotary portion.
The radial clearance between the balancing drum and bushing arrangement, which however cannot be lowered under a set minimum value, greatly affects the axial leakage, with a loss of a part of the pump flowrate, and a consequent reduction of the pump efficiency.